The structure of the stator and rotor of, for example, a prior art outer rotor type three-phase DC brushless motor will be described with reference to FIG. 3.
The structure of a stator 1 will be described. A laminated core in which electromagnetic steel plates are laminated with each other is used as a stator core 4. A plurality of pole teeth 4a projects radially outward from the annular core body. A coil (not illustrated) is wound around each of the pole teeth 4a. The stator core 4 is press-bonded and assembled to the outer periphery of a bearing housing (not illustrated). A bearing is fitted into the bearing housing.
A rotor 8 integrally assembled by swaging a cup-shaped rotor yoke 12 with respect to the rotor hub fixed to one end of the rotor shaft. Annular rotor magnets 13 multipole-magnetized in the circumferential direction are assembled to the inner peripheral surface of the rotor yoke 12. After the stator 1 is assembled, the rotor 8 is fitted by passing the rotor shaft through the bearing of the bearing housing. At this time, assembly is performed so that the pole teeth 4a of the stator core 4 face the rotor magnets 13.
When the rotor is held in a constant position in the state in which the motor is not excited, the holding torque needs to be increased. For example, in the outer rotor type DC brushless motor illustrated in FIG. 3, the pole teeth 4a of the stator core 4 need to face the magnetic poles of the rotor magnets 13 on a one-to-one basis. However, since the disposition of the stator core 4 so as to face the rotor magnets 13 does not necessarily form a magnetic circuit, there is proposed a technique for increasing the holding torque by preparing an additional core other than the stator core 4 and causing the pole teeth to face the magnetic poles of the rotor magnets 13 on a one-to-one basis (see PTL 1: JP-A-2015-89327).